001// Copyright 2007, 2008, 2011 The Apache Software Foundation
002//
003// Licensed under the Apache License, Version 2.0 (the "License");
004// you may not use this file except in compliance with the License.
005// You may obtain a copy of the License at
006//
007//     http://www.apache.org/licenses/LICENSE-2.0
008//
009// Unless required by applicable law or agreed to in writing, software
010// distributed under the License is distributed on an "AS IS" BASIS,
011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
012// See the License for the specific language governing permissions and
013// limitations under the License.
014
015package org.apache.tapestry5.ioc.services;
016
017import org.apache.tapestry5.ioc.AnnotationProvider;
018import org.apache.tapestry5.ioc.ObjectLocator;
019import org.apache.tapestry5.ioc.ObjectProvider;
020import org.apache.tapestry5.ioc.annotations.UsesOrderedConfiguration;
021
022/**
023 * A service that acts as a chain-of-command over a number of {@link org.apache.tapestry5.ioc.ObjectProvider}, but
024 * allows for the case where no object may be provided.
025 * <p/>
026 * This service is itself a key part of Tapestry's general injection mechanism; it is used when instantiating a service
027 * implementation instance, invoking module methods (service builder, decorator, or contribution methods), when
028 * {@linkplain ObjectLocator#autobuild(Class) autobuilding} objects of any type.
029 * <p/>
030 * As of Tapestry 5.3, the MasterObjectProvider allows injection of {@link org.apache.tapestry5.ioc.OperationTracker} as a
031 * special case (not based on a contributed ObjectProvider).
032 */
033@UsesOrderedConfiguration(ObjectProvider.class)
034public interface MasterObjectProvider
035{
036    /**
037     * Provides an object based on an expression. The process of providing objects occurs within a particular
038     * <em>context</em>, which will typically be a service builder method, service contributor method, or service
039     * decorator method. The locator parameter provides access to the services visible <em>to that context</em>.
040     * <p/>
041     * When the value is required and no {@link ObjectProvider} provided a non-null value, then {@link
042     * ObjectLocator#getService(Class, Class[])}  is invoked (with no marker annotations),
043     * to provide a uniquely matching service, or throw a failure exception
044     * if no <em>single</em> service can be found.
045     *
046     * @param objectType         the expected object type
047     * @param annotationProvider provides access to annotations (typically, the field or parameter to which an
048     *                           injection-related annotation is attached); annotations on the field or parameter may
049     *                           also be used when resolving the desired object
050     * @param locator            locator for the <em>context</em> in which the provider is being used
051     * @param required           if true (normal case) a value must be provided; if false then it is allowed for no
052     *                           ObjectProvider to provide a value, and this method may return null to indicate the
053     *                           failure
054     * @param <T>
055     * @return the requested object, or null if this object provider can not supply an object
056     * @throws RuntimeException if the expression can not be evaluated, or the type of object identified is not
057     *                          assignable to the type specified by the objectType parameter
058     */
059    <T> T provide(Class<T> objectType, AnnotationProvider annotationProvider, ObjectLocator locator, boolean required);
060}